High bit rate communications networks are becoming more popular with the proliferation of computers with fast digital ports, such as Gigabit-Ethernet (“GigE”). In many situations it is desired to extend a local network link to an adjacent building or to a metropolitan fiber optics link available for access across the street or a few hundred meters away. When the installation of fiber optics for the desired link is hampered by economical or regulatory obstacles, a wireless solution is a preferred alternative. Some links use free-space optics (FSO), however the vulnerability of the link to fog makes it a poor choice in many locations, and a radio-frequency solution is preferred.
A promising spectrum allocation for wideband radio links applications is the high millimeter band, in the 50 GHz to 100 GHz range, and more specifically the 60 GHz band and similar bands which are offered by governments for license-free operation or easy licensing arrangements and fewer spectral-use constraints than those in lower bands. Because of the high directivity of antennas at this frequency and the absorption of oxygen and water vapor in some of these bands, each link is essentially private, free of interference from other links, thus some of the interference rejection measures can be relaxed in a radio design for cost reduction. The availability of several GHz per band is highly suitable for wideband yet low cost applications, because simple modulation schemes can be used, utilizing the wide bandwidth to gain cost reduction.
A low cost radio at these high frequencies requires several simplifications compared to digital radio links typically in use today. The radio must be easy to install and align by less experienced personnel, preferably with a skill set similar to those who install satellite television antennas. The antenna alignment mechanism should be intuitive to point to a desired direction and should obtain a fraction of a degree in angular precision without the need for electrical alignment aids. Since the cost of a millimeter-wave radio is highly affected by the cost of the front-end circuitry, that part of the system should be minimized in complexity.
One technique that has been utilized to simplify a radio front-end complexity is the use of the transmitter as a local oscillator for down conversion of the received signal. The separate local oscillator circuit is eliminated, having fewer components at high frequency. Such scheme requires transmit-modulation cancellation in the receiver. The modulation for the transmitted signal is delayed and subtracted at the right amplitude from the received signal. Such schemes have been disclosed in U.S. Pat. Nos. 4,134,068, 4,238,850 and 4,520,474. While these techniques reduce the front-end component count, they require precise replication of the transmitted signal after being modulated. Since modulators are non-linear and temperature dependents, the cancellation circuitry may require signal recovery feedback loops or other compensation circuitry that may add significantly to the complexity and cost it intended to reduce. It is desired to benefit from modulation cancellation simplicity without precision signal-domain subtraction. It is further desired to reduce the size and number of active components in a radio front-end without significant sacrifice in performance.
Thus, it is desirable to provide a wideband digital radio with transmit modulation cancellation in accordance with the invention that overcomes these limitations of the typical systems.